Convection
This figure shows a calculation for thermal
convection in the Earth's mantle. Colors closer to
red are hot areas and colors closer to blue are in
warm and cold areas. A hot, less-dense lower
boundary layer sends plumes of hot material
upwards, and likewise, cold material from the top
moves downwards.
Convection is the heat transfer due to the
bulk movement of molecules within
fluids such as gases and liquids,
including molten rock (rheid). Convection
includes sub-mechanisms of advection
(directional bulk-flow transfer of heat),
and diffusion (non-directional transfer of
energy or mass particles along a
concentration gradient).
Thermal image of a newly lit Ghillie kettle. The
plume of hot air resulting from the convection
current is visible.
Convection cannot take place in most
solids because neither bulk current flows
nor significant diffusion of matter can
take place. Diffusion of heat takes place
in rigid solids, but that is called heat
conduction. Convection, additionally may
take place in soft solids or mixtures
where solid particles can move past each
other.
Thermal convection can be
demonstrated by placing a heat source
(e.g. a Bunsen burner) at the side of a
glass filled with a liquid, and observing
the changes in temperature in the glass
caused by the warmer fluid circulating
into cooler areas.
Convective heat transfer is one of the
major types of heat transfer, and
convection is also a major mode of mass
transfer in fluids. Convective heat and
mass transfer takes place both by
diffusion – the random Brownian motion
of individual particles in the fluid – and
by advection, in which matter or heat is
transported by the larger-scale motion of
currents in the fluid. In the context of
heat and mass transfer, the term
"convection" is used to refer to the
combined effects of advective and
diffusive transfer.[1] Sometimes the term
"convection" is used to refer specifically
to "free heat convection" (natural heat
convection) where bulk-flow in a fluid is
due to temperature-induced differences
in buoyancy, as opposed to "forced heat
convection" where forces other than
buoyancy (such as pump or fan) move
the fluid. However, in mechanics, the
correct use of the word "convection" is
the more general sense, and different
types of convection should be further
qualified, for clarity.
Convection can be qualified in terms of
being natural, forced, gravitational,
granular, or thermomagnetic. It may also
be said to be due to combustion,
capillary action, or Marangoni and
Weissenberg effects. Heat transfer by
natural convection plays a role in the
structure of Earth's atmosphere, its
oceans, and its mantle. Discrete
convective cells in the atmosphere can
be seen as clouds, with stronger
convection resulting in thunderstorms.
Natural convection also plays a role in
stellar physics.
The convection mechanism is also used
in cooking, when using a convection
oven, which uses fans to circulate hot air
around food in order to cook the food
faster than a conventional oven.
Terminology
The word convection may have slightly
different but related usages in different
scientific or engineering contexts or
applications. The broader sense is in
fluid mechanics, where convection refers
to the motion of fluid regardless of
cause.[2][3] However, in thermodynamics
"convection" often refers specifically to
heat transfer by convection.[4]
Examples and applications
Convection occurs on a large scale in
atmospheres, oceans, planetary mantles,
and it provides the mechanism of heat
transfer for a large fraction of the
outermost interiors of our sun and all
stars. Fluid movement during convection
may be invisibly slow, or it may be
obvious and rapid, as in a hurricane. On
astronomical scales, convection of gas
and dust is thought to occur in the
accretion disks of black holes, at speeds
which may closely approach that of light.
Heat transfer
A heat sink provides a large surface area for
convection to efficiently carry away heat.
Convective heat transfer is a mechanism
of heat transfer occurring because of
bulk motion (observable movement) of
fluids.[5] Heat is the entity of interest
being advected (carried), and diffused
(dispersed). This can be contrasted with
conductive heat transfer, which is the
transfer of energy by vibrations at a
molecular level through a solid or fluid,
and radiative heat transfer, the transfer of
energy through electromagnetic waves.
Heat is transferred by convection in
numerous examples of naturally
occurring fluid flow, such as wind,
oceanic currents, and movements within
the Earth's mantle. Convection is also
used in engineering practices of homes,
industrial processes, cooling of
equipment, etc.
The rate of convective heat transfer may
be improved by the use of a heat sink,
often in conjunction with a fan. For
instance, a typical computer CPU will
have a purpose-made fan to ensure its
operating temperature is kept within
tolerable limits.
Convection cells
Convection cells in a gravity field
A convection cell, also known as a
Bénard cell is a characteristic fluid flow
pattern in many convection systems. A
rising body of fluid typically loses heat
because it encounters a colder surface.
In liquid, this occurs because it
exchanges heat with colder liquid
through direct exchange. In the example
of the Earth's atmosphere, this occurs
because it radiates heat. Because of this
heat loss the fluid becomes denser than
the fluid underneath it, which is still
rising. Since it cannot descend through
the rising fluid, it moves to one side. At
some distance, its downward force
overcomes the rising force beneath it,
and the fluid begins to descend. As it
descends, it warms again and the cycle
repeats itself.
Atmospheric convection
Atmospheric circulation
Idealised depiction of the global circulation on Earth
Atmospheric circulation is the largescale movement of air, and is a means by
which thermal energy is distributed on
the surface of the Earth, together with
the much slower (lagged) ocean
circulation system. The large-scale
structure of the atmospheric circulation
varies from year to year, but the basic
climatological structure remains fairly
constant.
Latitudinal circulation occurs because
incident solar radiation per unit area is
highest at the heat equator, and
decreases as the latitude increases,
reaching minima at the poles. It consists
of two primary convection cells, the
Hadley cell and the polar vortex, with the
Hadley cell experiencing stronger
convection due to the release of latent
heat energy by condensation of water
vapor at higher altitudes during cloud
formation.
Longitudinal circulation, on the other
hand, comes about because the ocean
has a higher specific heat capacity than
land (and also thermal conductivity,
allowing the heat to penetrate further
beneath the surface ) and thereby
absorbs and releases more heat, but the
temperature changes less than land. This
brings the sea breeze, air cooled by the
water, ashore in the day, and carries the
land breeze, air cooled by contact with
the ground, out to sea during the night.
Longitudinal circulation consists of two
cells, the Walker circulation and El Niño /
Southern Oscillation.
Weather
How Foehn is produced
Some more localized phenomena than
global atmospheric movement are also
due to convection, including wind and
some of the hydrologic cycle. For
example, a foehn wind is a down-slope
wind which occurs on the downwind side
of a mountain range. It results from the
adiabatic warming of air which has
dropped most of its moisture on
windward slopes.[6] Because of the
different adiabatic lapse rates of moist
and dry air, the air on the leeward slopes
becomes warmer than at the same
height on the windward slopes.
A thermal column (or thermal) is a
vertical section of rising air in the lower
altitudes of the Earth's atmosphere.
Thermals are created by the uneven
heating of the Earth's surface from solar
radiation. The Sun warms the ground,
which in turn warms the air directly
above it. The warmer air expands,
becoming less dense than the
surrounding air mass, and creating a
thermal low.[7][8] The mass of lighter air
rises, and as it does, it cools by
expansion at lower air pressures. It stops
rising when it has cooled to the same
temperature as the surrounding air.
Associated with a thermal is a downward
flow surrounding the thermal column.
The downward moving exterior is caused
by colder air being displaced at the top of
the thermal. Another convection-driven
weather effect is the sea breeze.[9][10]
Stages of a thunderstorm's life.
Warm air has a lower density than cool
air, so warm air rises within cooler air,[11]
similar to hot air balloons.[12] Clouds
form as relatively warmer air carrying
moisture rises within cooler air. As the
moist air rises, it cools, causing some of
the water vapor in the rising packet of air
to condense.[13] When the moisture
condenses, it releases energy known as
latent heat of condensation which allows
the rising packet of air to cool less than
its surrounding air,[14] continuing the
cloud's ascension. If enough instability is
present in the atmosphere, this process
will continue long enough for
cumulonimbus clouds to form, which
support lightning and thunder. Generally,
thunderstorms require three conditions
to form: moisture, an unstable airmass,
and a lifting force (heat).
All thunderstorms, regardless of type, go
through three stages: the developing
stage, the mature stage, and the
dissipation stage.[15] The average
thunderstorm has a 24 km (15 mi)
diameter. Depending on the conditions
present in the atmosphere, these three
stages take an average of 30 minutes to
go through.[16]
Oceanic circulation
Ocean currents
Solar radiation affects the oceans: warm
water from the Equator tends to circulate
toward the poles, while cold polar water
heads towards the Equator. The surface
currents are initially dictated by surface
wind conditions. The trade winds blow
westward in the tropics,[17] and the
westerlies blow eastward at midlatitudes.[18] This wind pattern applies a
stress to the subtropical ocean surface
with negative curl across the Northern
Hemisphere,[19] and the reverse across
the Southern Hemisphere. The resulting
Sverdrup transport is equatorward.[20]
Because of conservation of potential
vorticity caused by the poleward-moving
winds on the subtropical ridge's western
periphery and the increased relative
vorticity of poleward moving water,
transport is balanced by a narrow,
accelerating poleward current, which
flows along the western boundary of the
ocean basin, outweighing the effects of
friction with the cold western boundary
current which originates from high
latitudes.[21] The overall process, known
as western intensification, causes
currents on the western boundary of an
ocean basin to be stronger than those on
the eastern boundary.[22]
As it travels poleward, warm water
transported by strong warm water
current undergoes evaporative cooling.
The cooling is wind driven: wind moving
over water cools the water and also
causes evaporation, leaving a saltier
brine. In this process, the water becomes
saltier and denser. and decreases in
temperature. Once sea ice forms, salts
are left out of the ice, a process known
as brine exclusion.[23] These two
processes produce water that is denser
and colder. The water across the
northern Atlantic ocean becomes so
dense that it begins to sink down through
less salty and less dense water. (The
convective action is not unlike that of a
lava lamp.) This downdraft of heavy, cold
and dense water becomes a part of the
North Atlantic Deep Water, a southgoing
stream.[24]
Mantle convection
An oceanic plate is added to by upwelling (left) and
consumed at a subduction zone (right).
Mantle convection is the slow creeping
motion of Earth's rocky mantle caused by
convection currents carrying heat from
the interior of the earth to the surface.[25]
It is one of 3 driving forces that causes
tectonic plates to move around the
Earth's surface.[26]
The Earth's surface is divided into a
number of tectonic plates that are
continuously being created and
consumed at their opposite plate
boundaries. Creation (accretion) occurs
as mantle is added to the growing edges
of a plate. This hot added material cools
down by conduction and convection of
heat. At the consumption edges of the
plate, the material has thermally
contracted to become dense, and it sinks
under its own weight in the process of
subduction at an ocean trench. This
subducted material sinks to some depth
in the Earth's interior where it is
prohibited from sinking further. The
subducted oceanic crust triggers
volcanism.
Stack effect
The Stack effect or chimney effect is the
movement of air into and out of
buildings, chimneys, flue gas stacks, or
other containers due to buoyancy.
Buoyancy occurs due to a difference in
indoor-to-outdoor air density resulting
from temperature and moisture
differences. The greater the thermal
difference and the height of the structure,
the greater the buoyancy force, and thus
the stack effect. The stack effect helps
drive natural ventilation and infiltration.
Some cooling towers operate on this
principle; similarly the solar updraft tower
is a proposed device to generate
electricity based on the stack effect.
Stellar physics
An illustration of the structure of the Sun and a red
giant star, showing their convective zones. These
are the granular zones in the outer layers of these
stars.
Granules—the tops or upper visible sizes of
convection cells, seen on the photosphere of the
Sun. These are caused by the convection in the
upper photosphere of the Sun. North America is
superimposed to indicate scale.
The convection zone of a star is the
range of radii in which energy is
transported primarily by convection.
Granules on the photosphere of the Sun
are the visible tops of convection cells in
the photosphere, caused by convection
of plasma in the photosphere. The rising
part of the granules is located in the
center where the plasma is hotter. The
outer edge of the granules is darker due
to the cooler descending plasma. A
typical granule has a diameter on the
order of 1,000 kilometers and each lasts
8 to 20 minutes before dissipating.
Below the photosphere is a layer of much
larger "supergranules" up to 30,000
kilometers in diameter, with lifespans of
up to 24 hours.
Cooking
A convection oven is an oven that has
fans to circulate air around food, using
the convection mechanism to cook food
faster than a conventional oven.[27]
Convection ovens distribute heat evenly
around the food, removing the blanket of
cooler air that surrounds food when it is
first placed in an oven and allowing food
to cook more evenly in less time and at a
lower temperature than in a conventional
oven.[28] A convection oven has a fan
with a heating element around it. A small
fan circulates the air in the cooking
chamber.[29][30]
Mechanisms
Convection may happen in fluids at all
scales larger than a few atoms. There are
a variety of circumstances in which the
forces required for natural and forced
convection arise, leading to different
types of convection, described below. In
broad terms, convection arises because
of body forces acting within the fluid,
such as gravity.
The causes of convection are generally
described as one of either "natural"
("free") or "forced", although other
mechanisms also exist (discussed
below). However, the distinction between
natural and forced convection is
particularly important for convective heat
transfer.
Natural convection
This color schlieren image reveals thermal
convection from a human hand (in silhouette) to the
surrounding still atmosphere.
Natural convection, or free convection,
occurs due to temperature differences
which affect the density, and thus relative
buoyancy, of the fluid. Heavier (denser)
components will fall, while lighter (less
dense) components rise, leading to bulk
fluid movement. Natural convection can
only occur, therefore, in a gravitational
field. A common example of natural
convection is the rise of smoke from a
fire. It can be seen in a pot of boiling
water in which the hot and less-dense
water on the bottom layer moves
upwards in plumes, and the cool and
more dense water near the top of the pot
likewise sinks.
Natural convection will be more likely and
more rapid with a greater variation in
density between the two fluids, a larger
acceleration due to gravity that drives the
convection or a larger distance through
the convecting medium. Natural
convection will be less likely and less
rapid with more rapid diffusion (thereby
diffusing away the thermal gradient that
is causing the convection) or a more
viscous (sticky) fluid.
The onset of natural convection can be
determined by the Rayleigh number (Ra).
Note that differences in buoyancy within
a fluid can arise for reasons other than
temperature variations, in which case the
fluid motion is called gravitational
convection (see below). However, all
types of buoyant convection, including
natural convection, do not occur in
microgravity environments. All require
the presence of an environment which
experiences g-force (proper
acceleration).
Forced convection
In forced convection, also called heat
advection, fluid movement results from
external surface forces such as a fan or
pump. Forced convection is typically
used to increase the rate of heat
exchange. Many types of mixing also
utilize forced convection to distribute one
substance within another. Forced
convection also occurs as a by-product
to other processes, such as the action of
a propeller in a fluid or aerodynamic
heating. Fluid radiator systems, and also
heating and cooling of parts of the body
by blood circulation, are other familiar
examples of forced convection.
Forced convection may happen by
natural means, such as when the heat of
a fire causes expansion of air and bulk
air flow by this means. In microgravity,
such flow (which happens in all
directions) along with diffusion is the
only means by which fires are able to
draw in fresh oxygen to maintain
themselves. The shock wave that
transfers heat and mass out of
explosions is also a type of forced
convection.
Although forced convection from thermal
gas expansion in zero-g does not fuel a
fire as well as natural convection in a
gravity field, some types of artificial
forced convection are far more efficient
than free convection, as they are not
limited by natural mechanisms. For
instance, a convection oven works by
forced convection, as a fan which rapidly
circulates hot air forces heat into food
faster than would naturally happen due
to simple heating without the fan.
Gravitational or buoyant
convection
Gravitational convection is a type of
natural convection induced by buoyancy
variations resulting from material
properties other than temperature.
Typically this is caused by a variable
composition of the fluid. If the varying
property is a concentration gradient, it is
known as solutal convection.[31] For
example, gravitational convection can be
seen in the diffusion of a source of dry
salt downward into wet soil due to the
buoyancy of fresh water in saline.[32]
Variable salinity in water and variable
water content in air masses are frequent
causes of convection in the oceans and
atmosphere which do not involve heat, or
else involve additional compositional
density factors other than the density
changes from thermal expansion (see
thermohaline circulation). Similarly,
variable composition within the Earth's
interior which has not yet achieved
maximal stability and minimal energy (in
other words, with densest parts deepest)
continues to cause a fraction of the
convection of fluid rock and molten
metal within the Earth's interior (see
below).
Gravitational convection, like natural
thermal convection, also requires a gforce environment in order to occur.
Granular convection
Vibration-induced convection occurs in
powders and granulated materials in
containers subject to vibration where an
axis of vibration is parallel to the force of
gravity. When the container accelerates
upward, the bottom of the container
pushes the entire contents upward. In
contrast, when the container accelerates
downward, the sides of the container
push the adjacent material downward by
friction, but the material more remote
from the sides is less affected. The net
result is a slow circulation of particles
downward at the sides, and upward in
the middle.
If the container contains particles of
different sizes, the downward-moving
region at the sides is often narrower than
the largest particles. Thus, larger
particles tend to become sorted to the
top of such a mixture. This is one
possible explanation of the Brazil nut
effect.
Solid-state convection in ice
Ice convection on Pluto is believed to
occur in a soft mixture of nitrogen ice
and carbon monoxide ice. It has also
been proposed for Europa,[33] and other
bodies in the outer solar system.[34]
Thermomagnetic convection
Thermomagnetic convection can occur
when an external magnetic field is
imposed on a ferrofluid with varying
magnetic susceptibility. In the presence
of a temperature gradient this results in a
nonuniform magnetic body force, which
leads to fluid movement. A ferrofluid is a
liquid which becomes strongly
magnetized in the presence of a
magnetic field.
This form of heat transfer can be useful
for cases where conventional convection
fails to provide adequate heat transfer,
e.g., in miniature microscale devices or
under reduced gravity conditions.
Capillary action
Capillary action is a phenomenon where
liquid spontaneously rises in a narrow
space such as a thin tube, or in porous
materials. This effect can cause liquids
to flow against the force of gravity. It
occurs because of inter-molecular
attractive forces between the liquid and
solid surrounding surfaces; If the
diameter of the tube is sufficiently small,
then the combination of surface tension
and forces of adhesion between the
liquid and container act to lift the liquid.
Marangoni effect
The Marangoni effect is the convection
of fluid along an interface between
dissimilar substances because of
variations in surface tension. Surface
tension can vary because of
inhomogeneous composition of the
substances or the temperature-
dependence of surface tension forces. In
the latter case the effect is known as
thermo-capillary convection.
A well-known phenomenon exhibiting
this type of convection is the "tears of
wine".
Weissenberg effect
The Weissenberg effect is a
phenomenon that occurs when a
spinning rod is placed into a solution of
liquid polymer. Entanglements cause the
polymer chains to be drawn towards the
rod instead of being thrown outward as
would happen with an ordinary fluid (i.e.,
water).
Combustion
In a zero-gravity environment, there can
be no buoyancy forces, and thus no
natural (free) convection possible, so
flames in many circumstances without
gravity smother in their own waste
gases. However, flames may be
maintained with any type of forced
convection (breeze); or (in high oxygen
environments in "still" gas environments)
entirely from the minimal forced
convection that occurs as heat-induced
expansion (not buoyancy) of gases
allows for ventilation of the flame, as
waste gases move outward and cool, and
fresh high-oxygen gas moves in to take
up the low pressure zones created when
flame-exhaust water condenses.[35]
Mathematical models of
convection
Mathematically, convection can be
described by the convection–diffusion
equation, also known as the generic
scalar transport equation.
Quantifying natural versus forced
convection
In cases of mixed convection (natural
and forced occurring together) one
would often like to know how much of
the convection is due to external
constraints, such as the fluid velocity in
the pump, and how much is due to
natural convection occurring in the
system.
The relative magnitudes of the Grashof
number and the square of the Reynolds
number determine which form of
convection dominates. If
,
forced convection may be neglected,
whereas if
, natural
convection may be neglected. If the ratio,
known as the Richardson number, is
approximately one, then both forced and
natural convection need to be taken into
account.
See also
Bénard cells
Convection oven
Churchill–Bernstein equation
Combined forced and natural
convection
Double diffusive convection
Fluid dynamics
Heat transfer
Heat conduction
Thermal radiation
Radiation properties
Heat pipe
Laser-heated pedestal growth
Nusselt number
Thermomagnetic convection
Vortex tube
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Archived 2008-03-17 at the
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